1. Field of the Invention
The present invention pertains to the light-emitting diode, and more particularly to the package of a light-emitting diode with an electrostatic protection element.
2. Description of the Prior Art
Generally, the light-emitting diode has the characteristics of: small volume, lower power consumption, longer life time, short response time, and with excellently monochromatic color. Generally, it is found to be applied in the home appliance, computer and its periphery, and communication products. Since, 1993 Nichia Chemical corp., successfully developed the gallium nitride (GaN), the blue light-emitting diode, which enables the light-emitting diode with fully colors to be realized and thus expands applications thereof to the fully color display, traffic light signal, traffic information panel and instrument panel in car, the braking light, and rear side-marker light. According to the research reports, the high intensity LED such as tour elements AlGaInP LED with wavelength ranges from yellow-green light to red light can reduce the chip number furthermore. The day of using LED instead of tungsten lamp to attain the purpose of lower maintenance and save electricity consumption seems to come approaching.
In general, the GaN blue light emitting diode is grown on the sapphire substrate, which is an insulator. Thus the n-electrode and p-electrode thus are required to be formed on the same side and the chip could not be too small, the size of a chip is typically about 350 μm×350 μm. However, the P/N junction is very close to the surface, and thus is readily destroyed by the electrostatic charges. Particularly, under dry environment, the electrostatic charges are easily accumulated to a level of about 1-2 KV on the human body. Under such situation, if it happens that the man unintentionally touches one of the diode pins, even a minute current may still destroy the light-emitting diode, which is typically operate at the range of 1-4 volt. In particular the blue light or blue-green light-emitting diode has the highest merchandise price among the various light emitting diode because the owners who have the manufacture technique are rare and high price of sapphire substrate. All of the factors support the price of the blue light or blue-green LED to be several folds or even hundred folds of others among three primitive colors. Thus, the package of the blue or blue-green LED associates with electrostatic element is very crucial.
Currently, to prevent the light-emitting diode from the electrostatic discharge, the LED is in parallel with a protective element. FIG. 1 shows a schematic circuit of the light-emitting diode is shunt with a Zener diode 2. The blue light or blue-green light-emitting diode 1 is operated at a voltage of forward biased between about 3-4V The Zener diode 2 is worked reverse biased, typically at a voltage of Zener breakdown, which is about 8V. In normal operation, the operation voltage is lower than Zener breakdown, as a result, no electricity power is consumed from the Zener diode because the off-state. However, in case of high voltage such as 1000V-2000V, accumulated by electrostatic charges is touched on any pin of the LED, will cause the LED device and Zener diode both turn on, however, the current is almost drained through the Zener diode 2, which is in Zener breakdown because of much low impedance In consequent, the light-emitting diode 1 is being protected.
There are several of conventional methods proposed to construct the circuit as shown in FIG. 1. However, each of them though solve some disadvantages present in the prior art but new issues are associated with the newly method. For example, please see FIG. 2, a first embodiment, the package proposed by Inoue et al., in U.S. Pat. No. 6,333,522 may have the best brightness for a single blue light emitting diode as we had known. The Inoue's patent includes twelve embodiments. Most of them include only the minor structure modified on light-emitting diode or silicon diode. An exemplary one in them is shown please see FIG. 2 and FIG. 2A, the bottom of the Zener diode 2 having an n-electrode 9 is positioned on the flat bottom of the cone-shaped reflector 15 through a conductive silver paste layer 14. The reflector 15 designed is in accordance with the reflective angle of light from the light emitting element. Beneath flat bottom of the reflector 15 is a leadframe 13a, which connects to a positive terminal of DC (direct current). The Zener diode 2 includes a p-electrode 7 having a mini-bump 11 thereon, and a bonding pad 10 on the p region 21, which is in the n+ doped substrate 2, as well as an n-electrode 8 having a bump 12 on the n-type substrate 20. In addition, the light-emitting diode 1 having an n-electrode 6 and a p-electrode, respectively, mounted on the bump 11 and 12 so as to form electrical connections with the p-electrode 7 and n-electrode 8 of Zener diode 2. A gold wire 17 is then bonding to the bonding pad 10 of Zener diode 2 and the leadframe of the negative electrode 13b. Finally a transparent resin 18 as package material is then capsulated them to form the light-emitting diode entity.
For blue light LED is concerned, most of them are with the n-electrode 6 and the p-electrode 5 on one side due to the insulation of the sapphire substrate. Of course as the substrate is silicon carbide the n-electrode and the p-electrode may at different sides. For flip-chip as Inoue et al proposed, only one bonding wire 17 is required, the upward surface of the light-emitting surface is free from any bonding pad. As a result, as the light intensity is concerned, it gives most satisfied brightness among all LED packages.
However, to tell about the package process, several issues are found. Since the area of the light-emitting diode chip is about several tens mil square typically, is about 12 mil×12 mil (1 mil is about {fraction (1/1000)} inch), and the solder is about 30˜50 μm in height for a conventional bump process. Thus for a chip is flipped, the difficulty of alignment is drastically increased while the p-electrode 5 of the and n-electrode of the light-emitting diode, respectively, aligned with such miniature bumps 12 and 11 on the n-electrode 8 and the p-electrode 7 of Zener diode It will be detrimental to the mass produce and the yield. Worthwhile increasing the size of the solder bump is not appropriated since it will cause the risk of the circuit in short between the p-electrode 7 and the n-electrode 8. Furthermore, to avoid the silver paste 14 over filled, the Si-base substrate: the Zener diode can not be formed too thin in thickness. Typically the thickness is about 1500 μm˜200 μm. Still, the heat dissipation can only slowly dissipate through the light-emitting diode itself and part of them from the silicon diode. Therefore the package techniques provided by Inoue still have room to improve.
For the silicon diode manufacture technique is concerned, the p-region 21 formed in the n substrate 20 is through lithographic and doping processes. It will increase fabrication cost compared with those Zener diode, which has p-electrode and n-electrode on the different sides.
Please refer to FIG. 3, the second embodiment of conventional technique issued to Sonobe et al in U.S. Pat. No. 6,054,716. By contrast to first embodiment, the chip of the light-emitting diode and the chip Zener diode 55 are positioned at different heights. One is on the bottom 61 of the reflector, atop the leadframe of positive electrode 52a, the other one is on the flange 62 of the reflector. In the second embodiment the Zener diode 55 has a p-electrode and an n-electrode on different sides. The n-electrode 55a of the silicon Zener diode is mounted on the flange 62 of the reflector through the silver paste 58. The p-electrode 55b of Zener diode 55 is then through conductive wire 68 connects to the leadframe of the negative electrode 52b. The p-electrode 65 of the light-emitting diode 53 is then connected to the leadframe of positive electrode 52a through a wire 66. The n-electrode 63 of the light-emitting diode 53 connected to the p-electrode 55b of the Zener diode 55 through a wire 67. Finally, the light-emitting diode 53 and Zener diode 55 and the reflector 61 along with the wires 66, 67 and 68 are molded with a transparent resin 73.
In the second embodiment, it require three wires (means two pads 65, 63 on the LED, and one on the Zener diode 55), the light intensity is thus weaker than the flip-chip type, which has one bonding wire on the Zener diode 1 merely. However, the yield and mass producing can be attained significantly improvement. Although the benefits as depicted above, some fatal problems are associated with the structure of the second embodiment: (1) the position height of the flange 62 and the bottom 61 of the reflector are different, and both of them are required to have a silver paste layer welded, and thus the welding stud machine would be much expensive than those for just one point stud. Furthermore, the area of the flange 62 is far less than the area of the bottom 61 of the reflector, As a result, the difficulty for pasting the silver paste thereon. In summary, it's still required to improve.
Please refer to FIG. 4 of a schematic diagram of the third embodiment according to the prior art. The present embodiment is in accordance with the U.S. Pat. No. 6,084,252 disclosed by Isokawa. A Zener diode 105 has an n-electrode (not shown) formed at its bottom face welded on a lateral position of a positive electrode of the leadframe 107a with a silver paste layer. A p-electrode formed at a top face of the Zener diode 105 electrically connects to a lateral surface of a negative electrode of the leadframe 107b via conductive wire 104. A LED 103 mounted on a recess portion of a cone-shaped reflector 101 has a p-electrode 111 and an n-electrode 113. The p-electrode 111 and the n-electrode 113 respectively connect to the positive electrode of the leadframe 107a and to the negative electrode of the leadframe 107b of the reflector via conductive wires 108, 109. Finally, the respective tip portion of the leadframes 107a and 107B and the above element 103, are molded with transparent resin 116 to form a dome-shaped LED structure.
The package structure of the third embodiment can solve the problems of about misaligning the Zener diode 10 with the LED 20 according to the first embodiment, and can also solve the problems of silver paste welding difficulty on the flange 62 of the cone-shaped reflector according to the second embodiment. However, for practical welding process is concerned, to weld the silver paste layer stud on a predetermined position of to the lateral position of a leadframe 107a by robot arms would at least have to turn leadframe or turn robot arm by 90° with respect to the upright position. Therefore, it's unpractical technique unless the silver paste-welding machine is re-designed or re-equipped.
An object of the present invention is thus proposed a package design to raise the process yield as well as mass producing the light-emitting diode with an electrostatic protection device.